Wireless communication networks comprise a plurality of cells, each of which is covered by a base station (BS). With most networks, the base stations perform the mobility management functions that allow a given piece of User Equipment (UE), such as a mobile station (MS), to move between cells. Generally, each base station in each cell transmits a unique beacon or pilot signal. The mobile stations use these signals to identify the base stations and to measure the signal strength from each base station. The mobile stations then send this information to the base stations. Based on this information, the base stations make mobility decisions regarding which cell or cells should be used to support communications with a given mobile station.
Typically, each cell transmits a neighbor list to help the mobile stations identify new cells as it moves through the network. These neighbor lists exist in networks such as Global System for Communications (GSM) networks and Universal Mobile Telecommunications System (UMTS) networks. However, other communication networks may not employ neighbor lists. By way of example, Evolved-UMTS Terrestrial Radio Access Networks (E-UTRAN), also referred to as Third Generation Partnership Project UMTS Long Term Evolution (3GPP LTE) networks, will make neighbor list use optional. Therefore, these types of networks may need to rely on a mobile station's ability to detect cells without neighbor lists. There is a variety of ways to accomplish this.
With LTE networks, for example, the radio multiplexing method is based on Orthogonal Frequency Division Multiplexing (OFDM). Enhanced UMTS Terrestrial Radio Access Networks (E-UTRAN), which are “Re-Use One” systems, employ a similar approach. Re-use One systems are those in which each cell in the network uses the same frequency as it's neighbor cell (e.g., the same set of sub carriers in OFDM). This means that in LTE networks, the mobile stations will be able to read/receive signals transmitted by a given neighbor cell while the mobile station is located in another cell. However, interference from the current cell might be high.
With mobile station-based mobility in UTRAN networks, for example, the mobile station will read the neighbor list from system information message received from the mobile station's current cell. When the mobile station detects that another cell is better than its current cell (e.g., based on specific criteria defined in the standard and by parameters also sent on system information message), the mobile station will select the new cell if that cell is defined as a neighbor in the neighbor list. Depending on which Radio Resource Control (RRC) state the mobile station is in, the mobile station may send an update message to the network to indicate that it has moved to a new cell or state. If the mobile station is in the RRC IDLE state, it will not send any messages to the Radio Access Network (RAN) indicating a new location.
For network-controlled mobility, a mobile station normally gets a neighbor cell list from the network in a dedicated message. Based on conditions and criteria defined by the standard, as well as by parameters sent from the network, the mobile station triggers a measurement report for transmission to the network whenever it detects a neighbor cell. The network will use the content in the measurement report to determine whether the mobile station should be handed off to the new cell. If so, a handover message is sent to the mobile station ordering it to connect to the new cell. After handover, the network usually updates the mobile station's neighbor list to reflect the identities of the neighboring base stations of this new cell.
There are three main categories of neighbor cells. These are intra-frequency neighbors (i.e., when the neighbor cell center frequency falls within the mobile station reception bandwidth), inter-frequency neighbors (i.e., when the neighbor cell center frequency falls outside the mobile station's reception bandwidth, and inter-RAT neighbors (i.e., when the neighbor cell uses an access technology that is different from the access technology employed by the currently-serving cell.
The fact that E-UTRAN is a Re-use One system reduces the need to have neighbor lists listing explicit neighbors, as is done in GSM communication networks. For example, the mobile station will detect strong signals from neighbor cells having favorable radio conditions especially for the current frequency. The base stations in these cells, then, would become candidates for handover or cell-reselection.
Therefore, E-UTRAN networks rely on the mobile station for both mobile station-based and network-controlled mobility. Particularly, these networks rely on the mobile station to detect a set of neighboring cells. Existing technology also allows the mobile stations to apply specific parameter settings when performing a re-selection or a handover to certain cells. With GSM and UMTS networks, the parameters to be used by mobile stations are included as a part of the neighbor list.
The parameters required for E-UTRA networks are similar to those contained in the neighbor lists. Thus, for mobile station-based mobility, the parameters may be included in an optional neighbor list broadcast as a part of the current cell System information message. For network-controlled mobility, the parameters may be included in a dedicated Measurement Configuration message. Alternatively, each cell may broadcast its own specific parameters. The mobile station then reads neighboring cell system information message to acquire any specific parameters needed to effect handover to that cell.